Study On The Direct Catalytic Oxido-amination Of Benzene To Aniline

Aniline is a kind of important organic molecule which is widely used in industry. Current commercial methods for preparing aniline involve multiple reaction steps and generally suffer from low atom utilization, strict operation condition, large amount of by-products and serious environmental pollution, which could not afford for the needs of "green"chemistry and sustainable development. Direct activation of C-H bond of benzene to produce aniline is a challenging topic in chemistry and has attracted much attention recently. In the present work, benzene and aqueous ammonia are used as starting materials, hydrogen peroxide is used as the oxidant to synthesis aniline. A series of Ni-V catalysts were prepared by wet impregnation method and used for the catalytic oxido-amination of benzene. Aniline was successfully synthesized directly from benzene under mild reaction conditions (low temperature and atmosphere pressure) using batch reaction method and catalytic distillation method. The results of control experiment and blank experiment show that the target reaction is heterogeneously catalyzed one. Using Ni and V as the active components, γ-Al₂O₃ supported catalyst shows better activity than the CeO₂, TiO₂ and SiO₂ supported ones for the catalytic oxido-amination of benzene. A series of Ni-V/γ-Al₂O₃ catalysts with different metal loadings were prepared with different impregnation procedures. It is found that the catalytic results are closely related to the relative contents of the active species and the crystalline species formed in the catalyst. As the main catalytic component, Ni shows certain activity for the amination of benzene, but the mono-component Ni/γ-Al₂O₃ catalyst or V/γ-Al₂O₃ catalyst show no activity for aniline formation. The co-existence of Ni and V in the catalyst facilitates the formation of aniline which may be resulted from the synergetic effect of the co-existed Ni and V. The highest selectivity (85%) to aniline was obtained on the catalyst with a Ni/V molar ratio of 5.5, which was much higher than that to the by-product phenol. The optimal conditions obtained in the present work are as the following: reaction time, 2 hours; reaction temperature, about 333 K. Urea is used as one possible aminating agent for this one-step process and is investigated in various reaction media. It is found that the synergic effect between aqueous ammonia and urea under the conditions investigated is favorable for the production of aniline. The aniline yield was significantly increased from 3.2 mg/(5 mL C6H6) to 8.4 mg/(5 mL C6H6) by use of urea and ammonia as aminating agent. Based on the above results, the catalytic oxido-amination of benzene was conducted with a kind of process intensification technology---catalytic-distillation. The result shows that the aniline yield could be raised by this technology and is influenced by the packing method of the catalyst, the molar ratio of starting materials and the reaction time. The packing method of the catalyst in the catalytic distillation column influences the aniline yield more greatly. The best aniline yield obtained under the conditions investigated is 11 mg/(5 mL C6H6). The catalyst prepared in the present work is characterized by TPR and XRD techniques. Crystalline NiO species on Ni-V/γ-Al2O3 catalyst is found to be effective for the activation of C-H bond of benzene, N-H bond of aqueous ammonia and O-O bond of hydrogen peroxide and the co-existence of the species reducible at about 730 K and 830 K is favorable for the selective formation of aniline. The catalytic performance of the catalyst could be regenerated by reduction method and the catalyst could be reused.